Essentially, it started out as a mod for ArmA II where a zombie apocalypse has wiped out most life on earth, and it is now a standalone game running the same storyline.

You spawn with nothing but a torch and battery and have to, first of all, find food, drink & warm clothes so that you can survive. Once you've done that you can explore, loot weapons and gear from houses and defeat the odd remaining zombie along the way.
You can also come across people playing on the same server and they may be friendly and travel with you or stick you up (maybe even kill you) to steal what you have.
If you die you will spawn again with nothing and have to search for gear/get started again, which doesn't take too long but is a pain - so it's a good idea to see if you can find things to defend yourself with fairly early on.

The scenery is beautiful, ArmA ran on a very powerful engine so the light and landscape rendering looks lovely.

It's taken me 4-5 starts to really get going but I think I'm well on the way to being stable and able to explore far and wide.